This is a multi-center, open-label, randomized, parallel, controlled comparison of Flolan plus conventional therapy to conventional therapy alone in patients with severe, primary pulmonary hypertension. The primary objective of this study is to confirm the hypothesis that patients treated with Flolan have increased exercise capacity compared to patients treated with conventional therapy. Secondary objectives are to assess the effects of Flolan therapy on cardiopulmonary hemodynamics, survival, right ventricular morphology, clinical signs and symptoms of PPH, quality of life, safety and health economics.